1. Field of the Invention
The present invention generally relates to the field of hydrocarbon conversion processes and specifically relates to a method and apparatus for separation and stabilization of products of a hydrogen-consuming hydrocarbon conversion process.
2. Discussion of the Prior Art
The process of "hydrocracking" has been generally known as a hydrogen-consuming conversion process for the refining of kerosene fractions, middle-distillate fractions, light and heavy vacuum gas oils, light and heavy cycle stocks, etc. for the primary purpose of reducing the concentration of various contaminating influences contained therein.
More recently, petroleum technology has turned towards the hydrocracking of residual stocks, such as tower bottoms products, crude products, topped crude oils, crude oils extracted from tar sands, shale oil, etc. Common among all such products is the addition of hydrogen to the oil feed and its passage through a reactor, with subsequent washing and high pressure low temperature separation. Such a process is schematically illustrated in FIG. 1, in which oil feed 10 and reactant gas 12 feed reactor 14, where the hydrocarbon conversion process takes place. The reactant gas 12 comprises recycle gas 16 (mostly hydrogen) and hydrogen makeup gas 18. The output of reactor 14 is a converted hydrocarbon oil with excess hydrogen gas, ammonia and hydrogen sulfide gases dissolved therein. Wash water 20 is added to the output of reactor 14 in order to separate the ammonia and hydrogen sulfide gases from the oil. A low temperature high pressure separator 22 is utilized to separate the oil from the water containing dissolved ammonia and hydrogen sulfide and further to separate any excess hydrogen gas contained in the mixture. This excess gas 24 is supplied to a compressor 26 powered by a power source 28. Compressor 26 compresses the excess gas 24, such that it can be added in the form of recycled gas 16 to the reactor 14. If desirable, a gas bleed 30 can be provided for removing a portion of excess gas 24.
Typical temperatures and pressures in the low temperature separator are on the order of 110.degree. F. and 2450 psia. The excess gas 24 comprises on the order of 80 plus volume percent hydrogen, with the remainder comprising ammonia, hydrogen sulfide, and hydrocarbons.
The oil product 32, from the low temperature separator 22, must then have its gas component removed from the syncrude component, so that the vapor pressure of the liquid syncrude is low enough to enable it to be stored and/or shipped. At the same time, the gaseous component of product 32 must be sufficiently rectified so that it can be used as fuel gas.
FIG. 2 illustrates a typical separation and stabilization system for the processing of the hydrotreated effluent 32 from FIG. 1. The hydrotreated effluent 32 supplied at high pressure is flashed to a lower pressure in low pressure separator 34, with a gaseous phase output 36 being transmitted directly to absorber 38. The liquid phase output 40, from low pressure separator 34, is transmitted to stripper tower 42, whereupon gaseous products are further removed from the liquid phase. Gaseous products from the stripper tower 42 are condensed in air fin cooler 44 conducted into a surge tank 46 and then fed back to stripper tower 42 and/or to absorber 38. The liquid phase output 45 leaving stripper tower 42 has a portion which is heated in reboiler 48 and then resubmitted to the stripper tower in order to boil off any remaining gases contained in the stripper tower 42 liquid phase output.
The absorber 38, accepting gaseous phase output 36 from the low pressure separator 34 and liquid phase output 47 from stripper tower 42, also receives liquid phase output 50 from liquid cooled heat exchanger 52. A portion of liquid phase output 54 from absorber 38 is passed through reboiler 51 to enhance separation of any further dissolved gases in the liquid phase output 54. All gases supplied to or evolved in absorber 38 pass into gas output conduit 56. The liquid phase output 54 of absorber 38, which is not recirculated through reboiler 51 passes to debutanizer 58.
The gaseous phase output 60, from debutanizer 58, passes through an air fin cooler 61 and into surge tank 63. Gas from the surge tank 63 is supplied directly to gas output conduit 56 and liquid from the surge tank is reintroduced into the debutanizer 58. A portion of the liquid phase output 62, from the debutanizer 58, is passed through reboiler 57 and reintroduced into debutanizer 58 to further separate gas from the liquid phase output 62.
A portion of the liquid phase output 62 which is not recirculated, passes into lean oil still 64. In lean oil still 64 the gaseous output phase 66 is circulated through an air fin cooler 65 and into a surge tank 69. The liquid phase output 68 from the lean oil still surge tank 69 is supplied directly to the syncrude output line 49. The liquid phase output 70 from the lean oil still surge tank 69 is resubmitted to the lean oil still 64. A portion of the liquid phase output 72 is recirculated through reboiler 74 back into the lean oil still 64. The remainder of the liquid phase output 72 passes through air fin cooler 73 and a portion passes into the syncrude output line 49. However, a portion of liquid phase output 72 also passes through the liquid cooled heat exchanger 52, and from there passes as a liquid phase output into absorber 38.
Thus, it can be seen that a typical prior art hydrotreated oil separation and stabilizing system requires energy input in the form of at least four reboilers 48, 51, 57, 74, at least five coolers (air fin coolers 44, 61, 65, 73 and liquid cooled heat exchanger 52) and four separate towers 38, 42, 58, 64, in addition to a low pressure separator 34. It would be desirable to reduce the capital expenditure required of such prior art systems, while at the same time reducing the complexity of operation. It also would be further advantageous to eliminate, to the extent possible, the energy requirements of the reboilers, which may well be wasted energy, ultimately lost to the atmosphere. Thus, the conventional way of achieving product separation has been by means of a stripper tower, in some instances preceded by a low-pressure flash drum, plus a three-tower gas plant. Each of these four fractionating towers is heated by a reboiler, which might in some instances be stripping steam to the stripper tower, which also requires that the overheads or gaseous phase outputs be condensed and the products cooled at the output of the unit.